Process for making dialkyl pyridylphosphates

ABSTRACT

The title compounds have heretofore been prepared by reacting an alkali metal pyridinate with a dialkyl hydrogen phosphite and carbon tetrachloride. We have discovered that the process is substantially improved by adding a small but catalytic amount of a tertiary amine having a pKa of at least about 9.5. The presence of the tertiary amine greatly accelerates the reaction rate and substantially reduces the production of undesirable by-products, e.g. N-alkylpyridones.

I UHitE States Patent 1191 Wang et al.

[ Dec. 23, 1975 PROCESS FOR MAKING DIALKYL PYRIDYLPHOSPHATES [75] Inventors: Chun Shan Wang; Michael J. Mintz,

both of Midland, Mich.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: Sept. 3, 1974 [21] Appl. No.: 502,481

Katritzky et al., Journal of the Chemical Society, 1967 (b) pp. 758-761.

Pao, Ac ta Chimica Sinica, v01. 27, pp. 160-161, (Nov. 1961).

Primary ExaminerJohn D. Randolph Attorney, Agent, or Firm-L. Wayne White [57] ABSTRACT The title compounds have heretofore been prepared by reacting an alkali metal pyridinate with a dialkyl hydrogen phosphite and carbon tetrachloride. We have discovered that the process is substantially improved by adding a small but catalytic amount of a tertiary amine having a pKa of at least about 9.5. The presence of the tertiary amine greatly accelerates the reaction rate and substantially reduces the production of undesirable by-products, e.g. N-alkylpyridones.

9 Claims, No Drawings PROCESS FOR MAKING DIALKYL PYRIDYLPHOSPHATES BACKGROUND OF THE INVENTION pounds may be represented by the formula wherein X is fluoro, chloro or bromo; n is an integer of from 0 to 3, inclusive; and R is lower alkyl. Rigterink teaches that the phosphates are prepared from the Rigterink eta1., cited above, teach that the pryidinols react with alkali metals or protonated tertiary amines to give the corresponding'alkali or ammonium pyridi- 5 nate. Equation 3.

/ OH base-) ln Equation 3, the base may be, for example, an alkali metal (e.g. sodium) or a protonated tertiary amine.

Pao indicates in the Acta Chimic'a Sinica 27 (2), 160-161 that the dialkyl hydrogen phosphites react with carbon tetrachloride to give the corresponding -dia1ky1 phosphorochloridates. Equation 4.

Equation 5 represents a summation of the reaccorresponding alkali metal or ammonium pyridinates tions illustrated by Equations 1-4.

and dia1kylphosphorochloridates; as illustrated by Equation 1.

Nishimura et al. (U.S. Pat. No. 3,686,191) teach the generic reaction represented by Equation 6.

x v(Ro+ i -H R'-Ha1 base- (Eq. 6)

dinols exist as tautomers. Equation 2. 65

When the components used in Equations 1-5 are inserted into Equation 6, it becomes Equation 7.

CCl base) (Eq. 7)

ll n O-P-(-OR)2. cac1 base 'HCl Each example in the Nishimura et al. patent follows the stoichiometry represented by Equations 6 and 7 exactly. The reactions were conducted in a hydrocarbon solvent, such as benzene, and upon mixing of the reactants, the reaction mixture was maintained at reflux conditions for a period of several hours (e.g. 4 hours). Under these reaction conditions, N-substituted pyridones are formed in substantial amounts. The N- substituted pyridones are undesirable by-products which must be removed. Product purification is both time consuming and expensive,so there is an obvious advantage to any process which would substantially reduce or eliminate the production of such undesirable by-products.

SUMMARY OF THE INVENTION In the process of preparing the 0,0-dialkyl-O- pyridylphosphates (I) by reacting by contacting a. an alkali metal or ammonium pyridinate of the 2 formula (Xn e e wherein R has the aforesaid meaning, and c. carbon tetrachloride, the improvement consisting of conducting said process in the presence of a small but sufficient amount of a tertiary amine to catalyze the reaction between (a), (b) and (c), said tertiary amine having a pKa of at least about 9.5. The benefits realized by the improvedprocess over the prior art are higher reaction rates, improved product yields, and increased product purity. The N-alkyl pyridones are produced in only minor amounts, if at all. Further benefits are derived from the fact that the instant process is normally conducted at a substantially lower temperature than the prior art reactions. Thermal degradation of the phosphate products is thus diminished and, there is an energy savings as well.

DETAILED DESCRIPTION OF THE INVENTION The alkali metal or ammonium pyridinates, reactant (a), are a known classof compounds. They may be generated in situ, or in a separate reaction. However, it is advantageous to generate the reactant in a separate reaction. The most preferred reactants are those represented in (a) wherein M is a sodium ion.

The dialkyl hydrogen phosphites, reactant (b), are likewise a well known class of compounds. They correspond to the formula (RO-HP-H,

wherein each R is a lower alkyl radical of from 1 to 4 carbon atoms but is preferably methyl or ethyl and is most preferably methyl.

The amine catalysts are tertiary amines having a pKa of at least about 9.5. This is a physical property which is well known for most amines and is easily measured by conventional techniques. For example, a listing of amines and pKa values is found in the Handbook of Organic Structural Analysis, Yasuhide Yakawa (Ed.) W. A. Benjamin, Inc., N.Y., N.Y. (1965) pages 587-688. The amine catalysts are used in the instant process in small but catalytic amounts (preferably from about 2 to about 10 mole percent, based on (a), (b) or (c), whichever is the lesser). Representative examples of suitable such amines include trialkyl amines (e.g. trimethylamine, triethylamine, tri-n-propylamine, tri-nbutylamine, methyldiethylamine, and the like); aralkyldialkylarnines (e.g. benzyldiethylamine, benzyldipropylamine, and the like); 5- and 6- membered heter- 5 ocyclic amines (e.g. N-methylpiperidine, N-ethylpiperidine, N-methylpyrrolidine, N-n-butylpyrrolidine, N-benzylpyrrolidine, and other like); and other like tertiary amines.

The reaction may be conducted neat at any converiient temperature so long as the reaction between (a), (b) and (c) occurs and the phosphate products can be recovered. We have found it advantageous, however, to normally run the reaction at a temperature of from about 20 to about 40C (preferably between about 20 and about 30C) in excess carbon tetrachloride as the liquid reaction medium/reactant. Other inert solvents can be used (e.g. benzene, toluene, etc.). Further, we prefer to conduct the instant reaction under substantially anhydrous conditions since water reacts with the dialkyl hydrogen phosphites to produce the corresponding phosphoric acids. Such phosphoric acids in turn react with the amine catalysts and/or with the sodium pyridinates and/or with the phosphate products, the net result of which is a lower product yield.

The following examples further illustrate the invention.

EXAMPLE 1 Preparation of dimethyl (3 ,5 ,6-trichloro-2-pyridyl )phosphate 1 increments over a 15 minute period, during which time the temperature of the reaction mixture was maintained below 30C by external cooling. After the addition of dimethyl hydrogen phosphite was complete, the

reaction mixture was stirred at 25C for an additional 45 minutes. The reaction mixture was washed with water to remove sodium chloride and the carbon tetrachloride phase was concentrated under reduced pressure to approximately ml. After the concentrate was cooled at -l0C for 4 hours, the slurry was filtered and the solid dried under reduced pressure to give 56.0 g (91.4 percent) of a white crystalline solid having a melting point of 9l-92C. This material was identified by infrared spectroscopy as 0,0-dimethyl 0-(3,5,6-trichloropyridyl) phosphate. Product purity was greater than 99 percent with less than 0.05 percent of the undesirable by-product 3,5,6-trichloro-N-methyl-2-pyridone.

Tri-n-propylamine, tri-n-butylamine and diisopropylethylamine were used in an analogous series of experiments with similar good results.

EXAMPLE 2 Preparation of dimethyl (3 ,5 ,6-trichloro-2-pyridyl )phosphate Anhydrous sodium 3,5,64richloro-2-pyridinate (44.1 g,- 0,2 mole), of carbon tetrachloride (175ml), and N-methylpiperidine (2.0 g, 0.02 mole) were slurried together at C. Dimethyl hydrogen phosphite (23.5 g, 0.21 mole) was added to the stirred mixture while holding the reaction temperature between about 20 and about C. The reaction was very exothermic. The reaction mixture was washed twice with 150 ml portions of water. The organic phase was separated from the wash water and concentrated to approximately 70 ml. After the concentrate was cooled at l0C for about 4 hours, the slurry was filtered and the filtrate dried under reduced pressure. The dried product was a white crystalline solid (54.8 g, 89.4 percent yield) melting at 9l92C. The product was at least 99.1 percent pure and contained only about 0.02 percent 3,5,6-trichloro-N-methyl-2-pyridone.

In analogous reactions, N-methylpyrrolidine was used as the catalyst with similar good results.

To show the effect of the amines in the reaction, a series of experiments were conducted analogous to Example 1 except that no catalyst was present. At 25C, the reaction was very slow and took several days. At C, the reaction took approximately 6 hours and the reaction product was contaminated with about 4 percent by weight of N-methylpyridone. At 60C, the reaction took approximately 2 hours and the product was contaminated with about 10 percent by weight of N-methylpyridone.

Preparation of dimethyl 3,5,6-trichloro-2-pyridylphosphate by the Rigterink process and the Nishimura et al. processes was in each instance plagued by the concurrent production of N-methylpyridone in amounts of about 10 percent by weight.

From this data, one observes that the tertiary amines having a pKa of at least about 9.5 are effective in catalyzing the reaction and raising the purity of the product at the expense of an undesirable by-product.

What is claimed is:

1. In the process of preparing a compound of the formula n N O P-(OR) 2 wherein X is fluoro, chloro or bromo, n is an integer of from 0 to 3, inclusive, and R is lower alkyl,

by reacting by contacting undersubstantially anhydrous conditions a. an alkali metal or ammonium pyridinate of the formula n N M wherein M is an alkali metal and X and n have the aforesaid meaning, b. a dialkyl hydrogen phosphite of the formula 5. The process defined by claim 1 wherein said process is conducted at a reaction temperature of from about 20 to about 40C.

6. The process defined in claim 5 wherein said reaction temperature is from about 20 to about 30C.

7. The process defined by claim 1 wherein said process is conducted in carbon tetrachloride as the liquid reaction medium.

8. The process defined by claim 1 wherein said tertiary amine is a trialkylamine, an aralkyldialkylamine or a 5- or 6- membered heterocyclic amine.

9. The process defined by claim 7 wherein: X is chloro; n is 3; M* is a sodium ion; R is methyl or ethyl; said amine is triethylamine, tri-n-propylamine, tri-nbutylamine, diisopropylethylamine, N-methylpiperidine or N-methylpyrrolidine, and wherein said process is conducted at a reaction temperature of from about 20 to about 30C. 

1. IN THE PROCESS OF PREPARING A COMPOUND OF THE FORMULA
 2. The process defined by claim 1 wherein M is a sodium ion.
 3. The process defined by claim 1 wherein R is methyl or ethyl.
 4. The process defined by claim 1 wherein R is methyl.
 5. The process defined by claim 1 wherein said process is conducted at a reaction temperature of from about 20* to about 40*C.
 6. The process defined in claim 5 wherein said reaction temperature is from about 20* to about 30*C.
 7. The process defined by claim 1 wherein said process is conducted in carbon tetrachloride as the liquid reaction medium.
 8. The process defined by claim 1 wherein said tertiary amine is a trialkylamine, an aralkyldialkylamine or a 5- or 6- membered heterocyclic amine.
 9. The process defined by claim 7 wherein: X is chloro; n is 3; M is a sodium ion; R is methyl or ethyl; said amine is triethylamine, tri-n-propylamine, tri-n-butylamine, diisopropylethylamine, N-methylpiperidine or N-methylpyrrolidine, and wherein said process is conducted at a reaction temperature of from about 20* to about 30*C. 